The subject matter disclosed herein relates to tip clearance measurement and, more particularly, to optical tip clearance measurement in a coaxial counter-rotating helicopter rotor.
A helicopter with coaxial counter-rotating rotors generally has a pair of helicopter rotors mounted one above the other on concentric shafts. The rotors have the same axis of rotation but turn in opposite directions. Conventional coaxial helicopters, have articulated rotors and utilize large spacing between the blades. This large spacing causes increased drag and degraded performance in forward flight. Coaxial helicopters can be designed for high speed flight, however, and in these cases they may have rigid rotors and much closer spacing to reduce drag. The close proximity of the blades requires that a tip clearance measuring system be deployed so that pilots are alerted if they have trimmed or maneuvered the aircraft in such a way that clearance margins are unacceptable.
In similar applications, laser trackers for production blade whirlstand tracking have been developed and can be similar to tip clearance measuring systems for helicopters. A laser tracker typically has 3 laser beams and each laser beam has a dedicated lens and photodiode. The lasers bounce their light of off a reflector strip, which is suspended above the path of the blade. When the blade blocks the laser beam, the photodiode shows a loss of signal and the signal is conditioned so each time a beam is just cut a pulse is generated. The first two laser beams are parallel and the difference in time between the first pulse and the second pulse is inversely proportional to the speed of the blade. The third laser beam is mounted on an angle and the higher the blade is flying, the longer the distance between the second and third laser beams. With the accurate speed calculation from the cutting of the first two laser beams, the time between the second and third beams can be used as a means to calculate the distance between the beams. Knowing the angle and the distance between the second and third beams, the height of the blade can be calculated.
For a flying aircraft, such as a helicopter, it is impractical to recreate the laser tracking system in order to deploy the same as a tip clearance measuring system. The reflector strip cannot be suspended above the blades and the track of the blades above a fixed surface is less important to know that the clearance between the blades themselves.